This invention relates to the reclamation of cured polysulfide sealants. More particularly, it relates to the reversion of the cured sealant to the liquid polysulfide and the recycling of the product. Still more particularly, it relates to the reduction of the cured polysulfide by heating it with a mercaptan-terminated liquid polysulfide.
More than 95% of all polysulfide polymers are made by the reaction of sodium polysulfide with bis(chloroethyl) formal and small proportions of 1,2,3-trichloropropane, which provides branching sites for crosslinking during a subsequent curing step. In general, the principal reactions that take place may be summarized by the equations: ##STR1##
The preparation of the polysulfides is described by E. R. Bertozzi in Macromolecular Syntheses, p.35, Vol. 7, E. M. Fettes, ed., John Wiley & Sons, Inc., New York, (1979).
Solid polysulfide elastomers having terminal thiol groups and a molecular weight of about 80,000 (n is from about 300 to 1000) may be cured by oxidation to give products used in printing rolls, paint-spray hose, solvent hose, gaskets, and gas meter diaphragms. Liquid polysulfide polymers, on the other hand, also have terminal thiol groups but their molecular weight is much lower, i.e. from about 400 to about 8000 (n is from about 6 to 50). These liquid polysulfides are cured in place at the site of their use, which is mainly as sealants for double-pane insulating windows, in other building construction, for boat hulls and decks, for aircraft integral fuel tanks, and in aircraft construction.
Waste material comprising scraps of cured polysulfide sealants generated by large scale applicators of aircraft, window, and industrial grade sealants is fast losing a place to be dumped. The cost of landfill cells is continuously escalating. Some landfill operators are beginning to refuse such sealants because of their heavy metal content. Manufacturers of the liquid polysulfides that are precursors to the cured sealants are beginning to search for ways to alleviate the disposal problems faced by their customers.
Hydrogen sulfide at rather low concentrations is known to attack cured polysulfides and cause the surface to soften. Hydrogen sulfide is, however, a very dangerous poisonous gas. It would be preferable, moreover, to cause the cured material to change into a liquid usable once again as a sealant curable by an oxidative process.
The problem of reclaiming vulcanized rubber from scrap material was addressed by Sverdrup et. al. in U.S. Pat. No. 2,415,449 wherein it is taught that a mixture of ground scrap and up to 7% by weight of a reclaiming agent, i.e., an oxygen carrier, is treated at greater than 100.degree. C. under conditions such that oxygen is available, either from the agent, the atmosphere, or elsewhere. A reversible plasticity is achieved rapidly and the treatment is stopped by, for example, removing the reclaiming agent to avoid reversion. Mercaptans, organic sulfides, terpenes, and unsaturated ketones are named as the oxygen carrying reclaiming agent, i.e., one which is capable of reversible oxidation and reduction.
U.S. Pat. No. 4,425,389 (Schollhorn et. al.) teaches that by mixing solid mercaptoterminal polysulfide polymers with from 5 to 15% by weight of liquid mercaptoterminal polysulfide polymers, one can produce sealing compositions which remain soft at 50.degree. to 100.degree. C. to such a point that they may be pumped and easily sprayed, even though a hardening agent is present.
There remains a need for a method for reverting a cured polysulfide sealant to a liquid, recurable state so that the sealant may be recycled in such manner over and over again, thus reducing the demand for scarce landfill space.